Cross-linking by irradiation decreases the fatigue strength of UHMWPE. In addition, post-irradiation melting further decreases the fatigue strength of the UHMWPE. Radiation and melting also decrease the yield strength, ultimate tensile strength, toughness and elongation at break of UHMWPE.
Melting in combination with irradiation creates cross-links and facilitates recombination of the residual free radicals trapped mostly in the crystalline regions, which otherwise would cause oxidative embrittlement upon reactions with oxygen. However, cross-linking and the decrease in the crystallinity accompanying post-irradiation melting are thought to be the reasons for the decrease in fatigue strength, yield strength, ultimate tensile strength, toughness and elongation at break of radiation cross-linked and melted UHMWPE. Some or all of these changes in properties limit the use of low wear highly cross-linked UHMWPE to low stress applications. Therefore, a cross-linked UHMWPE with higher crystallinity is desirable for low wear and high fatigue resistance for high stress application that require low wear.
It is, therefore, desirable to reduce the irradiation-created residual free radical concentration in cross-linked UHMWPE without reducing crystallinity, so as to achieve high fatigue resistance for high stress application that require low wear. Alternative methods to melting can be used to prevent the long-term oxidation of irradiated UHMWPE to preserve higher levels of crystallinity and fatigue strength.
The effect of crystallinity on the fatigue strength of conventional UHMWPE is known. Investigators increased the crystallinity of unirradiated UHMWPE by high-pressure crystallization, which increased the fatigue crack propagation resistance of unirradiated UHMWPE by about 25% (Baker et al., Polymer, 2000, 41(2): p. 795-808). Others found that under high pressures (2,000-7,000 bars) and high temperatures (>200° C.), polyethylene grows extended chain crystals and achieves a higher crystallinity level (Wunderlich et al., Journal of Polymer Science Part A-2: Polymer Physics, 1969. 7(12): p. 2043-2050). High pressure crystallization may improve the fatigue strength of irradiated UHMWPE despite no significant changes in ultimate tensile strength (Pruitt et al., 7th World Biomaterials Congress, 2004. p. 538, Bistolfi, et al., Transactions, Orthopaedic Research Society, 2005. p. 248) through first melting than pressurizing. The crystallization behavior of not cross-linked or highly cross-linked polyethylene at high pressures through first pressurizing, then heating at the high pressures has not been determined.
Polyethylene undergoes a phase transformation at elevated temperatures and pressures from the orthorhombic to the hexagonal crystalline phase. The hexagonal phase can grow extended chain crystals and result in higher crystallinity in polyethylene. This is believed to be a consequence of less hindered crystallization kinetics in the hexagonal phase compared with the orthorhombic phase. One could further reduce the hindrance on the crystallization kinetics by introducing a plasticizing or a nucleating agent into the polyethylene prior to high-pressure crystallization. The polyethylene can be doped with a plasticizing agent, for example, α-tocopherol or vitamin E, prior to high-pressure crystallization. The doping can be achieved either by blending the polyethylene resin powder with the plasticizing agent and consolidating the blend or by diffusing the plasticizing agent into the consolidated polyethylene. Various processes of doping can be employed as described in U.S. application Ser. No. 10/757,551, filed Jan. 15, 2004, PCT/US/04/00857, filed Jan. 15, 2004, U.S. Provisional Application No. 60/541,073, filed Feb. 3, 2004, and PCT/US2005/003305, filed Feb. 3, 2005, the entireties of which are hereby incorporated by reference.
Reduction in adhesive/abrasive wear of ultra-high molecular weight polyethylene (UHMWPE) components can be achieved by decreasing the large-scale deformation ability of the polymer. Cross-linking by ionizing radiation is generally used for this purpose (see Muratoglu et al., J Arthroplasty, 2001, 16(2): p. 149-160; Muratoglu al., Biomaterials, 1999, 20(16): p. 1463-1470; and McKellop et al., J Orthop Res, 1999, 17(2): p. 157-167) with a concomitant decrease in strength (Oral et al., Biomaterials, 2005).
In order to increase the strength of UHMWPE, high pressure crystallization (HPC) of UHMWPE has been proposed (see Bistolfi et al. Transactions of the Orthopaedic Research Society, 2005, 240; Oral et al. Transactions of the Orthopaedic Research Society, 2005, p. 988, U.S. Provisional Application No. 60/541/073, filed Feb. 3, 2004, and PCT/US2005/003305, filed Feb. 3, 2005). High pressure crystallization of unirradiated GUR1050 UHMWPE at above 160° C. and 300 MPa yielded an approximately 70% crystalline UHMWPE, compared to 50-60% for conventional UHMWPE. This is due to a phase transition of the UHMWPE crystals from the orthorhombic to the hexagonal phase at high temperatures and pressures as discussed above. In the hexagonal phase crystals grow to larger sizes and crystallinity increases (see Bassett et al, J Appl. Phys., 1974, 45(10): p. 4146-4150).
While high pressure crystallization can be used to increase the strength of UHMWPE, it has been shown to decrease the wear resistance of unirradiated UHMWPE (see Bistolfi et al., Transactions, Orthopaedic Research Society, 2005, p. 240). It appears that the decrease in ductility accompanying high pressure crystallization may adversely affect the wear resistance.